Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to a sheet feeding control.
Description of the Related Art
Hitherto, a stepping motor is often used in an image forming apparatus as a driving unit for a system of feeding and conveying a sheet. Although the stepping motor is small in size and may be manufactured at low cost, a so-called non-synchronous phenomenon frequently occurs, in which rotation of a rotator cannot be synchronized with input of a pulse signal due to overload or rapid change in rotational speed. When the non-synchronous phenomenon occurs under a state in which the rotator is accelerated, the rotator falls out of synchronization with the pulse signal, which stops the sheet feeding and conveying operation. Therefore, in order to prevent occurrence of the non-synchronous phenomenon even in various usage conditions of a user, a stepping motor having a large output torque is sometimes selected for the image forming apparatus. Further, as another countermeasure against the non-synchronous phenomenon, for example, in Japanese Patent Application Laid-Open No. 2008-301625, there is proposed a technology of controlling a drive current of the stepping motor in accordance with the magnitude of the load. Further, for example, in Japanese Patent Application Laid-Open No. 2013-151340, there is proposed a technology of changing the control based on the detected position of the rotor of the stepping motor, to thereby enhance the output efficiency of the stepping motor.
In order to reduce the cost of a sheet feeding mechanism, some image forming apparatus do not include a sensor function for detecting presence or absence of a sheet placed on a plate. In such image forming apparatus, when no sheet is placed on the plate, the plate may directly drive a sheet feeding roller under a state of being urged to the sheet feeding roller. In this case, the maximum load is applied to the stepping motor serving as the driving unit. When the stepping motor falls out of step under this state, the plate is maintained in the state of being urged to the sheet feeding roller, and hence the plate cannot be restored to the normal state. In view of this, the image forming apparatus of this type employs a stepping motor having a high output torque, which does not fall out of step even at a position at which the plate is urged to the sheet feeding roller. However, employment of an expensive stepping motor causes increase in cost.